1. Field of the Invention
This invention relates to a method of producing an oxide superconductor, more particularly to a method of producing an oxide superconductor which increases the critical temperature at which superconductivity is exhibited by an oxide containing copper, bismuth, vanadium or other element having a redox charge.
2. Prior Art Statement
Following the discovery by J. G. Bednorz and K. A. Muller of the original oxide superconductor consisting primarily of copper, there have been developed a number of other oxide superconductors which, owing to the incorporation and combination of various metal oxides, exhibit superconductivity at higher critical temperatures.
The oxide superconductors discovered up to now include at least one element having a redox charge among their constituent metal elements. Moreover, it is known that the element or elements are simultaneously present in different charge states at different portions of one and the same superconductor and that where these different charge states are regulated by oxygen, the oxygen atoms are usually not present in an integer ratio to the metal atoms. It is therefore thought that this presence of oxygen atoms at a non-integer ratio, or the variation in valence number corresponding to the number of metal element atoms, has an effect on the characteristics of the superconductor and its critical temperature.
In the production of superconductors consisting predominantly of copper, therefore, substances exhibiting different charges, such as oxides of rare earth metals, alkaline earth metals, bismuth and the like, have been added to the copper to regulate its valence number.
It is also believed that in oxide superconductors containing an element or elements having a redox charge such as bismuth (V) or vanadium (IV), the element or elements has/have an effect on the critical temperature at which superconductivity is exhibited.
Thus while numerous attempts have been made to develop new superconductors, the lack of any theoretical guidelines for increasing the superconducting critical temperature gives researchers engaged in such development no choice but to rely on the experience they accrue through their work.
On the other hand it has been reported that a superconductor with a critical temperature of 40 K. was obtained by electrochemically oxidizing a lanthanum-copper-oxygen (La-Cu-O) superconductor in a aqueous solution at 0.45 V (vs a mercury oxide electrode). (J-C. Grenier et al., Physica C 1991 p139-144.)
When a superconductor precursor oxide is electrochemically processed in an aqueous solution, the electrolytic potential of the oxide becomes a highly important factor. This will be explained using a La-Cu-O oxide as an example.
If a potential exceeding the appropriate potential range on the positive electrode is applied to the oxide, the generation of oxygen at the oxide surface will become so vigorous as to produce cracks in or even disintegrate the oxide serving as an electrode. Far from achieving introduction of oxygen, the conductivity of the oxide will be reduced. If an excessive negative potential is applied to the oxide, oxygen production will decline and generation of hydrogen will be induced, so that the conductivity of the once superconductive oxide will decline to the point where it loses its superconductivity altogether.
Grenier et al. state that they oxidized the composition at 0.45 V (vs a mercury oxide electrode) in light of the pH potential diagram of copper. However, when the inventors electrolyzed a La-Cu-O oxide of the same composition at the same electrolytic potential, they found that the conductivity of the oxide was reduced and that the critical temperature, Tc, usually exhibited provided the oxide is a superconductor, was not found and that the broad split width on X-ray spectrograms usually exhibited by a La-Cu-O oxide superconductor with a high critical temperature, Tc, was also reduced.
The object of this invention is to provide a method for producing an oxide superconductor which effectively and reliably enables the critical temperature of the superconductor to be increased.